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The original low pass filter converts PWM to DC voltage.

You can see the nasty ripple on the ouptut in the oscillscope:

enter image description here

Sallen-Key improvement:

enter image description here

What exactly is happening with the Sallen-Key that makes it have less ripple?

Try to explain it by identifing the effect that causes the basic low pass filter to have a downward ripple on the falling edge of the PWM signal. Is it due to a capacitor C1 discharging?? If it is due to C1 where is the current going when the PWM is on a falling edge/ 0 volts?

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    \$\begingroup\$ The capacitor charges and discharges by the 100k resistor which smooths the ripple since it is a lowpass filter. The Sallen-Key filter has TWO resistor and capacitor filers so it reduces the ripple twice as much as ONE RC filter. The LC at the output of the opamp shorts the ripple frequency to ground. \$\endgroup\$ – Audioguru Dec 1 '20 at 18:33
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  • The RC filter in your first diagram has a "1st order response".

  • The Sallen Key op-amp filter has a "2nd order response".

This means, that for the same cut-off frequency (3 dB point), the 2nd order filter has a significantly better ability to attenuate high frequencies in the "stop-band": -

enter image description here

Image from here.

The red trace is a first order filter and the blue trace is a 2nd order filter and, at any arbitrary point in the spectrum above the 3 dB point, the 2nd order filter produces a much higher attenuation of unwanted AC signals such as your PWM waveform.

What exactly is happening with the Sallen-key that makes it have less ripple??

It's still there but to a much lesser extent.

For the 1st order circuit it is the capacitor charging and discharging via the input resistor. But, for the 2nd order filter there an identical cascaded RC filter that both significantly attenuates the ripple seen in the first order circuit and reshapes it to be more sinusoidal so, if you homed in on the 2nd order output waveform you would see a small sinewave type ripple voltage.

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    \$\begingroup\$ To piggyback on Andy Aka's answer - Plot the filter response for your first R-C filter, then plot the response for your Sallen Key filter. You might want to look for an on-line tool to do this. I know TI had one on their web site at one time. The difference in the attenuation of your ripple frequency of each filter should be apparent. \$\endgroup\$ – SteveSh Dec 1 '20 at 19:08

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